1. Field of the Invention
The present invention relates to a solid electrolytic capacitor having an anode element made of a valve metal, a dielectric layer formed on a surface of the anode element, and a cathode layer including an electrically conductive polymer layer and formed on the dielectric layer.
2. Description of the Background Art
Regarding a solid electrolytic capacitor having an anode element made of a valve metal, a dielectric layer formed on a surface of the anode element, and a cathode layer including an electrically conductive polymer layer and formed on the dielectric layer, the chemical oxidative polymerization method and the electrolytic oxidative polymerization method have conventionally been known as a method of forming the conductive polymer layer. Here, the chemical oxidative polymerization method refers to a method of producing a polymer by allowing an oxidizing agent to act on a monomer to chemically oxidize and polymerize the monomer, and the electrolytic oxidative polymerization method refers to a method of producing a polymer by passing an electric current through a solution containing a monomer to electrolytically oxidize and polymerize the monomer. Generally, the electrolytic oxidative polymerization method uses a more complicated manufacturing apparatus. However, control of conditions for polymerization by the electrolytic oxidative polymerization method is easier as compared with the chemical oxidative polymerization method. Therefore, an electrically conductive polymer excellent in electrical conductivity, mechanical strength and homogeneity is produced more easily by the former method.
In order to form the electrically conductive polymer layer forming a main portion of the cathode layer by means of the electrolytic oxidative polymerization method, a first cathode layer is formed on the dielectric layer and the electrically conductive polymer layer that is a second cathode layer is formed on the first cathode layer by the electrolytic oxidative polymerization method. The first cathode layer is formed of a manganese dioxide layer produced by pyrolysis of manganese nitrate or an electrically conductive polymer layer produced by the chemical oxidative polymerization method. The second cathode layer is formed using the first cathode layer as an anode and a metal plate as a cathode and passing an electric current through a solution containing a monomer and a substance serving as a supporting electrolyte as well as a dopant agent (electrolytic polymerization solution).
In the process step of forming the electrically conductive polymer layer by the electrolytic oxidative polymerization method, as the polymerization is continued or repeated in the electrolytic polymerization solution, the monomer is consumed and the supporting electrolyte is also consumed by being taken as a dopant into the polymer. As the supporting electrolyte, a metal salt of an aromatic sulfonic acid for example is used. In the case where the anion of the supporting electrolyte salt is taken as a dopant into the polymer, it is called p-type doping. In the case where the cation of the supporting electrolyte salt is taken as a dopant into the polymer, it is called n-type doping. Generally it is considerably difficult to accomplish the n-type (cation) doping and thus usually the p-type doping is widely employed.
As for the p-type doping, the anion is taken as a dopant into the polymer. Therefore, the cation is left in the electrolytic polymerization solution to cause the pH of the solution to gradually change to become more basic. This results in the harmful effects that the electrical conductivity of the produced electrically conductive polymer is decreased and the ESR (Equivalent Series Resistance) of the resultant electrolytic capacitor is increased. On the contrary, the n-type doping causes the cation to be taken as a dopant into the polymer. Thus, the anion is left in the electrolytic polymerization solution and the pH of the solution is changed to gradually become acidic. In this case as well, some harmful effects could be caused.
As a solution to the problems as described above, the technique of adding such a pH adjusting agent as sulfuric acid to the electrolytic polymerization solution is disclosed in Japanese Patent Laying-Open No. 11121279.
However, even when the method of adjusting the pH by adding the sulfuric acid is used, continued use of the electrolytic polymerization solution causes the sulfate anion concentration in the electrolytic polymerization solution to cumulatively increase, which makes it difficult to control the ratio between aromatic sulfonic acid ions and sulfate ions taken as dopants into the polymer. Further, degradation of the electrolytic polymerization solution becomes worse and the ESR of the electrolytic capacitor increases. Therefore, the whole solution has to be replaced regularly.